Voltage source converters (VSC) are power switches comprising a plurality of semiconductor chips such as e.g. insulated gate bipolar transistor (IGBT) power modules. They are often used in high-voltage direct current (HVDC) applications for converting direct current to alternating current and vice-versa or in static var compensators (SVC) for reactive power compensation in power transmission systems.
Semiconductor chips such as IGBTs and gate turnoff (GTO) thyristors are suitable for high power applications. IGBTs are often preferable as they combine great power handling ability with features that make them well suited for connection in series and/or in parallel.
Short circuit situations may occur in PE modules. In such situations it is necessary to be able to handle the effect of the short circuit. When a semiconductor breaks down, e.g. as a result of an over current or over voltage, the semiconductor cannot hold a voltage any longer. A damaged semiconductor cannot be controlled. It may hold only a small voltage difference and when conducting its resistance can have a value within a broad range. In the worst case, forcing a current through a damaged semiconductor with high resistance can generate an arc that will generate extensive power dissipation.
One example of a power electronics module is described in U.S. Pat. No. 7,538,436 B2. Inside the module, semiconductor chips are arranged in parallel in order to increase the total current capability. The power electronics module has the ability to go into a stable short circuit failure mode (SCFM) in case of an IGBT or diode failure (due to e.g. cosmic radiation). When another IGBT or diode failure occur the module again enters an SCFM. This is called the SCFM transition. The PE modules are connected in series to obtain a valve with high blocking voltage. The blocking capability is taken over by the remaining modules in the stack in case of a short circuit in a module.
Accordingly, it is desirable to obtain an improved control of the SCFM transitions in power electronics modules.